1. Field of the Invention
The present invention relates to a portable data processing apparatus.
2. Description of the Prior Art
Data processing operations associated with portable data processing apparatus, such as music players, mobile telephones or games machines, can be controlled in response to motion of the whole apparatus.
Motion is often detected in portable apparatus using an accelerometer. A typical accelerometer is a small micro electro-mechanical systems (MEMS) device comprising a sprung cantilever beam with a mass supported at the free end of the beam. Damping of the motion of the beam and mass is provided by residual gas sealed in the device. Under the influence of an external acceleration (e.g. an acceleration of the apparatus in which the accelerometer is provided), the mass deflects from a neutral position. This deflection is measured using known analogue or digital techniques, for example by detecting the capacitance between one or more fixed electrodes and one or more beams attached to the mass.
The use of accelerometers is cheap and reasonably reliable. However, they suffer from the drawback that the results obtained from the accelerometer (i.e. a signal or value indicative of acceleration at a certain instant) must be integrated in order to obtain a signal or value indicative of speed, and this must in turn be integrated in order to obtain a signal or value indicative of position of the apparatus.
Similar factors apply to the detection of rotational motion (leading to a detection of orientation) using gyroscopes. The gyroscope's output must be integrated in order to obtain a signal or value indicative of current orientation.
Integration processes can tend to introduce drift in the output signal or value, because any errors or noise in the original data are also integrated, so that their effect on the accuracy of the output signal or value increases over time.
This is not a particular problem in some situations. For example, certain music players allow the user to change to a new music track or change replay volume by making a sudden flicking movement of the music player. The detection of such a movement is not particularly affected by problems of signal integration—in fact, the raw acceleration information can be tested directly to detect that type of movement.
However, in applications such as portable computer games machines, sometimes the current position and/or orientation of the machine is used as a control input to the game. For example, the machine could be treated as a type of steering wheel, so that movements of the machine cause changes in direction of a car within the game environment. Or the machine could act as a type of gun sight, so that movements of the machine change the aim of a gun in the virtual environment. In instances like these, a tendency for the motion detection to drift or, more generally, to be incorrect would be subjectively disturbing for the machine's user.
It is an object of the present invention to mitigate or alleviate the above problems.